Heat treatment of steel



Patented June 24, 193% PAT VIILLIAM B. SULLIVAN, OF DETROIT, MICHIGAN, ASSIGNOE TO CI-IROBALTIC TOOL COMPANY, OF DETROIT, MICHIGAN, A CORPORATION OF "ILLINOIS HEAT TREATMENT OF STEEL No Drawing. Application filed. June 2,

This invention relates to steel and to the processes of heat treatment thereof, and has for its principal object to improve the quality and increase the life of heat treated articles made of steel.

The method of heat treatment now in general use is based upon the critical temperature curves, known as the decalescent, or upper curve and the recalescent, or lower curve. The method consists of three steps. First, heating quickly to the upper or critical point of decalescent curve A0 3; second, cooling quickly as by quenching in oil or water, and, third, drawing or tempering to reduce the hardness to suit the conditions of the particular service.

High speed steel thus treated has certain well known imperfections. For example, only a thin layer has the proper structure characteristics hence, in the case of a cutting tool, as soon as this thin layer is ground away it becomes necessary to again heat treat the tool. Furthermore, the granules of this hardened area lack uniformity in size and in character. Sometimes the extreme outer surface will give a good cutting edge, and again this portion will have to be ground away to reach the granules of a different character lying below. There is often an extreme marked difference in cutting or other physical performance of tools or other articles made of steel originating in the same heat or cut from the same bar. 011 the whole, therefore, this generally used process and its products are not satisfactory.

This has been due first to a misconception that it was necessary to heat the steel to a temperature above that actually required to permit the transformation, and, second, to the fact that steel rapidly deteriorates if allowed to remain at the high temperature to which it is usually subjected. The transformation begins as soon as the decalescent curve is reached, but it is not instantaneous and requires time to proceed through the mass of the article under treatment. Soaking the steel at a temperature above the lower point in the decalescent curve, but below the upper point, the transformation beginning at the surface will progress form state without 1926. Serial mfiiasav.

through the mass of the steel and yet the transformed portion will not deteriorate. Thus, by soaking for a sufficient length of time at the proper temperature, the entire mass of the article can be brought into a uni- 55 detriment to any portion of it. Then by rapidly cooling the steel, as in the old process, the Austinitic or other condition, depending upon the character of material, can be maintained or fixed and by subsequently drawing, the hardness can e tempered as before to suit the particular requiren'ients of service. 7 l

The general character of st ucture upon quenching, as in operation No. 2, will be, depending upon the analysis and character of the material, Austinitic, Martensitic, or troostitic, or two-or more in combination. The general properties sought in steels are uniformity of crystallization and grain size 7 in the most finely divided state, it being the accepted principle that small grains are superior to large grains as manifested by lncrease in physical properties and resistance to shock and fatigue.

Holding the steel at a temperature above the lower point in the decalescent curve, but below the upper point for a suflicientlength of time produces the most finely divided grains and permits the grains to absorb their complete constituency from the enveloping matrix, and the matrix is reduced to an extremely thin coating or binder in which the grains are set and held.

The effect of the improved process is advantageously illustrated by casting dies and other tools for hot operations made of standard 18-4-1 high speed steel (18% tungsten 4% chromium and 1% vanadium). Such dies when treated in the usual manner will, after a period of service ranging from a thousand to five thousand castings, develop a surface known as alligator skin, because of the peculiar formation of ridges and valleys. This is due to the presence of relatively thick layers or coatings of relatively soft matrix on and around the hard grain lVhen such tools are treated according to the improved process the alligator skin condltion is prevented and the life of the dies, or other tools, is prolonged many times.

By the use of the improved process all those variations due to the history of the constituents, the diii'erences in working, the different parts of the ingots, etc, can be accounted for and the heat treated articles given the same uniform character. The characteristic differences in the two methods may be illustrated by a standard chromevanadium tool steel. B the old method this was quickly heated to 1575 F. and quenched in water; by the improved method the same hardness values with superior structure is obtained by soaking for four hours at LL35 F. (which, in the case oi this at el. is a point slightly above the A0 2 point in the transformation curv and then uenching it.

'lhe'length of the soaking period will vary with dirl'erent steels and with different pieces of the same analysis. The temperature at which the soaking is done will, of course, vary with diiierent steels.

For example, the temperature at which a particular high speed steel should be treated may be determined in the following manner: Selec three pieces hree-eighths inch square, heat one of them for period of one hour at 2215" E, quench it in oil and draw it for one and one-half hours at 1050 F; heat the second piece at 2250 F. for one hour, quench it in oil, and draw it for one and one-half hours at 1100 E; heat the third piece at 9.275" F. for one hour, quench it in oil and draw it ior one and one-half hours at 1150 F. After drawing, the three pieces can be cooled in air or in oil. By examination oi these specimens under the microscope and testing them in actual cutting on a standard test specimen, it will be determined that approximately one ot' the temperatures mentioned is correct, or that an interpolation between them, or slightly above or below the maximum mentioned, should be tried.

W here pyrometric examination is possible, the approximate temperature can be readily determined at once and the exact temperature can be ascertained by a little experiment.

The period of soaking should be extended until on examination of subsequent specimens the transformation is found completed throughout. Since no damage results from soaking at this relatively low temperature it is not essential that the steel be removed as soon as the transformation is complete though, of course, it may, under some conditions, be economical to terminate the soaking at that time.

It is, of course, desirable that soaking take place in a protected atmosphere and that the temperature be maintained substantially uniform. These conditions may be met by packing the articles to be treated in a box made of heat resisting alloy, or the like, surrounding it with a suitable inert material and placing the box and its contents in a furnace capable of maintaining all of its parts at a uniform temperature throughout the necessary period.

I claim as my invention:

1. The process of treating steel which includes soaking the same at a temperature above the lowest point in the decalescent curve but below the upper point thereof for a comparatively long period of time to effect transformation and then cooling to preserve the steel in its transformed condition.

2. The process of treating steel which includes soaking the same at a temperature below the upper critical point and above the lower critical point thereof, until the transformation is complete, and then cooling to preserve the transformed condition.

In testimony whereof I attix my signature.

WVILLIAM B. SULLIVAN. 

